The present invention relates in general to a wireless communication system and communication equipment, and more particularly to a relatively slow frequency hopped wireless communication system and communication equipment in which a plurality of communication equipment hops the frequency at a timing synchronous with a communication frame produced by a control station.
In the wireless mobile communication system, it is necessary to carry out the high quality communication through a unstable radio channel. As one technology for improving the transmission quality of performance in the unstable radio channel, there is well known a spread spectrum.
The spread spectrum includes two methods of "a direct sequence" and "a frequency hopping". Out of them, the frequency hopping has (1) the advantage that it can sufficiently cope with the distance problems, (2) the advantage that the frequency diversity can be readily constructed, and so forth, and therefore, in particular, the application of the spread spectrum to the land communication is expected. Incidentally, such a spread spectrum is explained in an article of "Spread Spectrum Communication System" by Mitsuo Yokoyama, Scientific Technology Publishing Company, 1988 for example. The frequency hopping is described in detail in the above-mentioned article, pp.563 to 611.
In general, the wireless channel is poor in the transmission quality of performance as compared with the wired channel, and hence has a tendency that the errors occur burstly. Therefore, even if the random error correction code such as a BCH code which can be relatively readily put to practical use is simply applied to the wireless communication system, it is difficult to realize the effective error control. Then, if the frequency hopping is employed which has the frequency diversity effect in principle and also the error correction code such as the BCH code is used together therewith, it is expected that the measures for preventing effectively the burst error can be realized. However, in order to obtain the frequency diversity effect in the frequency hopping, the hopping interval needs to be made shorter than the length of the allowable burst error.
For the above-mentioned reasons, for the conventional frequency hopping, there is employed a fast frequency hopping in which a plurality of hoppings are carried out within one symbol time period, or a slow frequency hopping in which the hopping is carried out about one time within several symbol time periods. In any case, as for the conventional frequency hopping, there is employed in many cases the hopping method having the short hopping interval in which the duration of one carrier frequency is within several times as long as the symbol period. However, in those conventional frequency hoppings, when carrying out the high speed transmission, the relatively expensive frequency synthesizer which can switch the frequency at a high speed becomes the essential factor.
With those points as background, while having a suspicion that the frequency diversity effect is reduced since the hopping interval is increased up to about several tens to 100 msec, the slow frequency hopping by which the relatively inexpensive synthesizer can be applied has been put to practical use. That is, the slow frequency hopping has been made by paying attention to the characteristics of the indoor wireless channel in which even if the fading period is relatively long (about several tens msec or more) and the hopping interval is long, a certain degree of frequency diversity effect can be obtained. Then, the slow frequency hopping is applied to the wireless LAN system pointing to the indoor communication.
However, even in the slow frequency hopped communication system in which the hopping interval is relatively increased in such a way, both the electric power of the received signal and the phase of the carrier are unstable before and after the switching of the frequency, and hence the continuity thereof is not ensured. As a result, it is difficult to make the frame length equal to or longer than the hopping period. Therefore, as for the communication form, the slow frequency hopping is applied in general to the communication of a burst mode in which the transmission and reception of the data are repeatedly carried out in units of frame.
In this case, since the influence of the over head portion of the preamble or the like upon the frame efficiency is large, as for the modulation method, there is employed in general the modulation method in which the preamble length of the communication frame has only to be short, e.g., the frequency modulation method, as the constant envelope modulation, in which the coherent detection is unnecessary.
If the multi-level quadrature amplitude modulation (hereinafter, referred to as "a QAM" for short, when applicable) in which the frequency efficiency is high and the high speed transmission is possible is intended to be applied as the modulation method, an automatic gain control (hereinafter, referred to as "an AGC" for short, when applicable) circuit needs to be provided in order to fetch effectively the amplitude component of the modulated signal. In addition, a circuit for recovering a carrier becomes essential in order to carry out the coherent detection. In order to recover the carrier, the field which is used to measure the electric power of the received signal and to recover the carrier needs to be prepared in the preamble field of each communication frame, and hence it is accompanied by the increasing of the preamble length.
In general, in the communication system of the slow frequency hopping, it is difficult to make the frame length equal to or longer than the hopping period, and hence that system is suitable for the burst communication rather than the continuous communication. In addition, the head portion of the communication frame used in the burst communication has the preamble field provided thereat, and each receiving unit carries out the specific adjustment and the synchronous work for the various circuits which are required for the signal modulation of the data field subsequent to the preamble field, such as the gain control of the receiver, the recovery of the carrier and the timing recovery, on the basis of the received information from the preamble field. The length of the preamble field is independent of the modulation method itself of the signal in the data field.
On the other hand, as for the modulation method for carrying out the high speed communication with the limited frequency band, the multi-level modulation is effective. According to the multi-level modulation, since the number of bits which can be transmitted within a unit time period is increased, the frame length (time) can be shortened as compared with other modulation methods if the number of bits constituting the communication frame is the same as that of other modulation methods. However, if the multi-level modulation is employed, there arises a problem that the characteristics of the signal to noise ratio v.s. the bit error rate are degraded and also the interference resistance against the multipass and the noise source is reduced. In addition, if the frame length is increased, in general, the frame error rate is necessarily increased. Therefore, even if the multi-level modulation is employed, it is difficult to increase largely the number of bits of the frame while maintaining the frame error rate constant. Accordingly, the communication needs to be carried out with the short frame divided into small pieces.
For the reasons described above, by employing the multi-level modulation for the information transfer, the information transfer field in the frame is decreased, but the preamble which is independent of the modulation method is not decreased. Therefore, the ratio of the information transfer field included in the frame length is low, and hence the frame efficiency is not improved. As a result, even if the multi-level modulation is used in the slow frequency hopped communication system, the superiority of the frequency efficiency thereof can not be sufficiently utilized.
As for the typical example of the multi-level modulation, with respect to the 16QAM for example, an article of IEICE (The Institute of Electronics, Information and Communication Engineers) Technical Report entitled "Performance of 16QAM MODEM for the Digital Land Mobile Communication System", RCS88-62, January, 1989 reports that when the time constant of the AGC is made equal to or less than 10 msec at the transmission rate of 16 k symbol/sec, even the variation in the amplitude due to the modulation is compensated. For this reason, the time constant of the AGC needs to be set to a value equal to or larger than 160 symbols.
In the conventional AGC, the method of converging the level of the signal to the optimal signal level using the closed loop is employed. Therefore, in order to carry out the AGC operation, the preamble length needs to be set to a value which is much longer than 160 symbols as described above, e.g., a value in the range of about several hundreds to 1,000 symbols.
On the other hand, we consider the maximum frame length of the communication frame used in the typical burst communication. For example, in the case of the packet of the ethernet as the typical example of the burst communication, the maximum data length thereof is about 1,500 bytes. When that data is subjected to the 16 quadrature amplitude modulation, the packet size thereof becomes about 3,000 symbols. Assuming that the preamble of 500 to 1,000 symbols for example is added to the head portion of that data, the frame efficiency is in the range of 75 to 86%. However, actually, since the auxiliary bits which are necessary for the recovery of the carrier, the timing recovery, the training of an equalizer and the like need to be added to the preamble field, the frame efficiency is further reduced as compared with the above-mentioned numeric value. Therefore, in the case where the multi-level modulation is applied in the burst communication, how the preamble is shortened is the key to the effective utilization of the band.
In the 16 levels or more quadrature amplitude modulation as the typical example of the multi-level modulation, the coherent detection becomes essential. In order to carry out the coherent detection, the recovery of the carrier from the received signal and the recovery of the timing signal need to be carried out with a high accuracy.
Heretofore, the structure of a sort of coherent detection and demodulation circuit is explained in an article of "Beginning of Mobile Communication" (by Yoshihisa Okumura et al. and edited by The Institute of Electronics, Information and Communication Engineers), 1986, p.154. In addition, the recovery of the timing signal is explained in the above-mentioned article, p.115. According to the structure of the coherent detection and demodulation circuit disclosed in the above-mentioned article, it is shown that both the operation of recovering the carrier and the operation of recovering the timing signal are carried out simultaneously, and the signal pattern "10101010 . . . " is normally used as the timing signal.
However, according to the above-mentioned conventional methods, the convergence operation of the recovered carrier and the recovered timing clock signal is changed due to the frequency difference between the received carrier and the local oscillator signal, the initial value of the phase difference therebetween, or the initial state of the timing recovery circuit for example. Since the above-mentioned convergence situation is difficult to be obtained analytically, it is difficult to ensure that the coherent detection and demodulation circuit is converged surely and with a high accuracy in a certain preamble length.